1. Field of the Invention
This invention relates generally to fiber optic fluorescence sensing, and, more particularly, to a shutter system for minimizing background variation in a fiber optic laser induced fluorescence sensor (LIFS).
2. Description of the Related Art
In a fiber optic based LIFS probe, an energy field propagates through a fiber to an active region. In the active region, evanescent or distal field excitation leads to a fluorescent signal from an analyte outside the fiber. The fluorescent signal propagates back through the fiber and is used to estimate the concentration of the analyte. Typical uses for such probes include detecting hazardous waste and monitoring the contamination of a process stream.
When using fiber optic spectroscopic sensors, it is common and desirable to transmit the excitation laser beam and the returning spectroscopic signal through the same fiber. Often the excitation beam excites native fluorescence in the fiber. Because in many systems it is impossible to filter the native fiber fluorescence from the signal due to overlapping spectra, the native fiber fluorescence will be detected and must be subtracted from the signal of interest. In many fibers the native fluorescence will slowly diminish in time as a result of continuous exposure to excitation light. This time-varying signal is difficult to quantify and subtract properly from the signal of interest, and thus degrades system performance.
As discussed in Bruce J. Tromberg et al., "Fiber-Optic Chemical Sensors for Competitive Binding Fluoroimmunoassay," Analytical Chemistry 59, 1226-30 (1987), in systems using dye labeled chemicals as part of the detection process, a hand operated shutter has been used to prevent fluorescent dye photodegradation and to enhance fiber-to-fiber reproducibility. This description does not address the problem of time-varying native fluorescence, however.
As described in Robert A. Ogert et al., "Comparative Analysis of Toxin Detection in Biological and Environmental Samples," SPIE Vol. 2068, 151-158 (1994), one attempt to minimize the native fluorescence has been to photobleach fluorescent components within the fiber itself until the fiber has a stable background drift over the time required to make the measurement. This technique generally requires pre-exposing the fibers to excitation light for approximately ten minutes immediately prior to use before any measurement is made. The fibers cannot be pre-exposed during their fabrication process because the photodegradation is a reversible process and the native fiber fluorescence will recover during fiber storage. The need to expose fibers to excitation light for a substantial period of time immediately prior to their use imposes a stringent limit on the field portability and use of any instrument requiring the fibers. Furthermore, if the measurement periods will require more than several minutes, longer pre-exposure times are required.